1. Field of the Invention
The present invention generally relates to an image forming device, and more particularly to an image forming device having an image reader unit to read an original image and output image data therefrom, and a ground-tint detection unit to detect a ground-tint pattern in the original image based on the image data.
2. Description of the Related Art
In recent years, with improvements in image-processing and image-forming technology, it becomes possible to carry out even the image formation of a bill (bank note) using a color copier and distinguishing a copied bill from the real bill is difficult to the human eyes.
For this reason, in order to prevent illegal reproduction of special originals or confidential documents using copiers, various preventive measures have been proposed.
One method of distinguishing a special original or the like is to compare an input image data with a registered specific mark (or pattern data) by using the pattern matching method. If the specific mark is detected in the input image data, it is determined that the original is a special original with the specific mark provided to inhibit the copying of the original image.
On the other hand, also in the case of normal originals other than special originals, such as bills or securities, in the offices of companies or the like, there are many documents the copy inhibition of which is demanded from the viewpoint of the significance and security protection of the contents of such documents. Generally, the “secret” stamp or “copy inhibition” stamp is affixed to the documents the copy inhibition of which is demanded, in order to distinguish the special originals from the normal documents which can usually be copied.
Moreover, a copy inhibition method for which a ground-tint pattern is embedded in the background of a document is also proposed. This method is to create a ground-tint pattern which has a base region and a message region in the background of a paper document, which allows one of the two regions to be easily copied using the copying machine, and makes it difficult to reproduce the other of the two regions. When the document with the ground-tint pattern is copied, the message region appears on the image of the document.
For example, by making the message region contain the characters “copying inhibited”, the characters “copying inhibited” can be reproduced at the time of outputting the copy.
Japanese Laid-Open Patent Application No. 06-125459 discloses a copying machine with a special original discrimination unit. The special original discrimination unit comprises a memory unit, a marking unit, an image extraction unit, a comparison unit, and a control unit.
The memory unit is provided to store feature data according to the kind of special originals, such as bills or negotiable securities, the copying of which is inhibited. The marking unit performs a predetermined marking on a recording sheet. The image extraction unit extracts a feature image portion from the input image data. The comparison unit compares the feature image portion extracted by the image extraction unit, with the feature data of the memory unit to determine a similarity of the input image data. The control unit inhibits the copy operation when the similarity determined by the comparison unit is above a predetermined first similarity. The control unit controls the marking unit to perform the marking on the recording sheet when the similarity is below the first similarity and above a predetermined second similarity.
According to the copying machine with the above-mentioned special original distinction function, it is possible to inhibit the copying of the special original and prevent illegal copying thereof, without barring the copying of the normal original.
Japanese Laid-Open Patent Application No. 2004-200897 discloses an image processing device comprising an image reader unit, a discriminating unit, a connecting unit, and a control unit. The image reader unit is provided to read an original image. The discriminating unit extracts a ground-tint pattern from the image data output from the image reader unit, compares the ground-tint pattern with a reference ground-tint pattern, and determines whether the original image contains the image data with which transmission of the image data to an external device or printing of the image data to a recording sheet is inhibited. The connecting unit is provided to control connection of the discriminating unit and the external device. The control unit is provided to control the respective units of the image processing device.
In the copying machine with the special original distinction function of Japanese Laid-Open Patent Application No. 06-125459 and the image processing device of Japanese Laid-Open Patent Application No. 2004-200897, the determination as to whether the original read by the image reader unit is a special original is made depending on the number of black dots or the distribution of black dots etc which exist in the reference area used as the reference.
However, the number of the black dots which existing in the reference area varies with the reading resolution used when the original image is read. Moreover, if minute ground-tint patterns are formed on the original read by the image reader unit and a reading resolution needed for the image reader unit to detect such ground-tint patterns correctly is not used, it is difficult to recognize the ground-tint patterns in the special original and it is difficult to prevent the outputting of the image data of the special original.